In a power semiconductor module, one power semiconductor element deals with a high power of several kilowatts; therefore, excessive thermal stress is repeatedly applied to upper and lower joints of the power semiconductor element due to heat generation by the element, thereby causing cracks in the joints formed of solder or the like. In order to enhance reliability of these upper and lower joints of the power semiconductor element, a technology of mold-sealing the entirety of the module with thermoplastic or thermosetting resin has been used. As a form of power semiconductor module using this mold-sealing technology, there is a power semiconductor module as shown in FIG. 14, in which an insulating substrate 10 includes a ceramic plate 1 with high thermal conductivity on which a front-side electrodes 2a and a back-side electrode 2b are formed, a power semiconductor element 3 is joined and wire-connected to the front-side electrode 2a, and the module including this insulating substrate is molded with mold resin 60.
In a power semiconductor module molded including the insulating substrate as described above, two sets of a diode and a switching element that are usually used in an inverter circuit are mounted in a two-in-one structure, thereby enabling the wire-connection to be completed within the mold, so that inductance can be reduced and its electrical characteristics can be enhanced. However, the front-side electrode 2a on the joint surface of the power semiconductor elements of the insulating substrate 10 is split and patterned into at least two parts as shown in FIG. 15, whereas the back-side electrode 2b on the back side is not split; therefore, stress in the front side is no longer balanced with that in the back side, resulting in an increase in warpage of the insulating substrate 10. If the warpage of the insulating substrate 10 increases, when a heatsink is connected to the power semiconductor module using grease, the power semiconductor module must be pressed onto the heatsink with excessive pressure in order to ensure their tight contact. If the power semiconductor module is pressed onto the heatsink with such excessive pressure as described above, shearing stress is applied between the mold resin 60 and the ceramic plate 1 and the front-side electrode 2a, thereby causing a problem that cracking and separation of the mold resin occur.
In coping with a problem with mold-sealing in a two-in-one structure such as the above, a method of splitting the insulating substrate into two parts so as to suppress its warpage has been proposed (see Patent document 1).